Eap actuator and drive method

ABSTRACT

An electroactive polymer actuator comprises an electroactive polymer structure and a driver for providing an actuation drive signal. In one aspect a first drive signal with an overdrive voltage is used to change the charge of the electroactive polymer structure needed for switching the structure from one to another actuation state. When or after the electroactive polymer structure actuation is near or at the another actuation state, a drive voltage is used to bring to and hold the electroactive polymer structure at the actuated state. This temporary overdrive scheme improves the speed response without damaging the electroactive polymer structure.

FIELD OF THE INVENTION

This invention relates to electroactive polymer actuators and devices or systems including such actuators as well as to methods for driving such actuators. It further relates to a computer implemented invention for performing the methods.

BACKGROUND OF THE INVENTION

Electroactive polymer actuators are devices that can transform an electrical input to (mechanical) output such as e.g. force or pressure or vice versa. Thus EAP actuators can be used as mechanical actuators and, depending on the EAPs used, often also as sensors. To this end they comprise electroactive polymers (EAP) which can deform or change shape under the influence of an actuation stimulus or signal. Some examples of field-driven EAPs include Piezoelectric polymers, Electrostrictive polymers (such as PVDF based relaxor polymers) and Dielectric Elastomers, but others exist.

EAP actuators can be easily manufactured into various shapes allowing easy integration into a large variety of systems such as for example medical or consumer devices. Further, EAP based actuators/sensors combine high stress and strain with characteristics such as: low power, small form factor, flexibility, noiseless operation, accurate operation, the possibility of high resolution, fast response times, and cyclic actuation.

Typically, their characteristics render an EAP actuator useful for e.g. any application where little space is available and in which a small amount of movement of a component or feature is desired, based on electric actuation. Similarly, the technology can be used for sensing small movements.

FIGS. 1 and 2 show two possible operating modes for an exemplary EAP actuator. It comprises an EAP structure including an EAP layer 14 sandwiched between electrodes 10, 12 on opposite sides of the EAP layer 14. FIG. 1 shows an actuator which is not clamped by (attached to) any carrier layer or substrate. A drive voltage applied to the electrodes is used to cause the EAP layer to expand in all directions as shown. FIG. 2 shows an actuator which is designed so that the expansion arises only in one direction. In this case a similar EAP structure as the one of FIG. 1 is supported and clamped, i.e. mechanically attached to a carrier layer 16. A voltage applied to the electrodes is again used to cause the EAP layer to expand in all directions as indicated for FIG. 1. However, the clamping confines the actual expansion such that a bowing of the entire structure is caused instead. Thus, the nature of this bending movement arises from the interaction between the passive carrier layer and the active layer which expands when actuated.

It appears that when an EAP actuator, such as the one of FIGS. 1 and 2, is activated using an electrical driving scheme, the actual desired mechanical actuation response deviates from the desired with respect to timing and/or actuation state. For example, a certain time delay between start of driving and reach of a desired actuation state occurs. This discrepancy hampers application of EAP devices as e.g. fast and accurate mechanical response is difficult.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved EAP actuator with respect to the hereinbefore mentioned discrepancy.

This object is at least partly achieved by the invention as defined by the independent claims. The dependent claims provide advantageous embodiments.

The device and method of the invention use an actuator that comprises an electroactive polymer structure for providing a mechanical actuation output dependent on a drive signal supplied to it. Such signal can be applied using electrodes. Thus, a first voltage makes the structure attain a first actuation state while a second voltage different from the first voltage makes it attain a second actuation state different from the first actuation state. The electroactive polymer structure comprises an electroactive polymer (EAP) to this end which is capable of changing its shape upon application of the drive signal. Examples of such EAP structures are described with reference to FIGS. 1 and 2, but others exist and the invention is not limited to these examples.

The invention is based on the insight that an EAP structure has an electrical impedance including a capacitance of the capacitor defined by the structure's EAP and electrodes and/or other layers configuration. The mechanical actuation of the EAP structure is dependent on the electrical field within and thus electrical charge on the effective capacitor. This is because the actuator is either electric field driven and such a field is dependent on charge, or is current driven, where, again, the current is caused by an electric field. Therefore, the ‘intrinsic’ speed of switching of the actuator is thus dependent on the charging speed of this effective capacitor. Due to the impedance a capacitive charge, needed to reach a certain mechanical actuation position of the EAP structure, requires time to build up to a specific level. Such a charging level is therefore only reached after a certain multiple of a characteristic time constant. Usually, after a time duration of 5 times this characteristic time constant, the maximum actuation extension (or end position) is reached for a specified drive signal. For applications where a fast response is needed with a limited delay time, or a higher frequency of operation is required, this behaviour might be a disadvantage and would hinder the usage of EAP actuators.

The invention thus employs a drive signal that comprises:

an overdrive period in which the drive signal voltage is changed from the first voltage to an overdrive voltage by an amount that exceeds the difference between the second voltage and the first voltage; and

after the overdrive period, a holding period at least at the beginning of which the drive signal voltage is at the second voltage.

The difference is always calculated by subtracting the first voltage value (including sign) from the second voltage value (including sign). Hence the voltage change and its direction is also indicated by the difference, being an increase of voltage for switching to second voltage higher than first voltage and a decrease of voltage for switching from a higher first voltage to lower second voltage. In the holding period the voltage has at least reached the second voltage at its beginning. It may be kept constant, but may also change as indicated below in order to hold the second actuation state constant.

With the invention charging or discharging to a desired state of the actuator can be controlled to reduce the delay in reaching that desired actuated state, and this can widen the scope of potential applications of EAP actuators. The invention also provides that actuation levels can be reached with still good accuracy, i.e. predetermined actuation overshoot or even devoid of substantial actuation overshoot.

The first actuation state can be of a higher actuation level that the second actuation state or vice versa. The first actuation state or second actuation state during a switching action can be the rest actuation state, i.e. the actuation state that is maintained by the actuator without any driving.

The first voltage or second voltage ideally are constant voltages. In that case an actuation state can be held by continuously providing its associated voltage. However, in less ideal cases (actuators) these voltages may (slowly) change. Thus a first voltage and/or second may be continuously or periodically changed over time to keep an actuator at its constant associated actuation state.

In the invention the second voltage can be applied only when or after the electroactive polymer structure reaches the second actuation state for the first time during the application of the electrical drive signal.

This may be feedback controlled or not.

In the invention the second voltage can be maintained at least as long as the overdrive voltage, or at least twice as long as the overdrive voltage. Thus, the actuation is essentially a low frequency driving of the actuator, in the sense that the complete actuation is much longer that the overdrive duration, and the voltage across the electroactive polymer structure stabilizes. The holding period can be at least 5 times as long, at least 10 times as long or even at least 50 times as long as the overdrive period.

In the invention the electroactive polymer structure can comprise electrodes for receiving the drive signal, the electrodes defining a capacitor having a capacitance, and

the overdrive voltage is applied until the voltage across the capacitance differs from the second voltage by a predetermined amount and changes to the second voltage subsequently. The predetermined amount can be less than a value chosen from the group consisting or: 50%, 20%, 10%, 5%, 2%, 1%, 0%. Hence a fast increase of the voltage across the capacitance to a value close to the second voltage can be achieved, giving a switching speed gain. If the predetermined amount is 0%, then the speed gain is optimal for the overdrive voltage waveform used.

The overdrive voltage can be applied until the voltage across the capacitance is at or below the second voltage. Thus, even though the overdrive voltage is larger than is needed to reach a desired actuation state, the capacitance of the electroactive polymer structure is never exposed to this voltage.

The overdrive voltage may be applied until the voltage across the capacitance exceeds the second voltage by the predetermined amount, such that the voltage changes back to the second voltage subsequently. This provides a temporary overdrive signal across the capacitance of the electroactive polymer structure. Since such overdrive voltage across the capacitance can be accomplished with mechanical overdrive, this embodiment may be used to provide an enhanced feeling to a haptic interface—felt as a short duration movement spike superposed on a more gradual vibration.

In the invention, during the overdrive period the overdrive voltage can be:

substantially constant, or

changing from an initial value to the second voltage.

The drive voltage may comprise a step increase to an initial value overdrive voltage and a subsequent change from this value to the second voltage. This provides a more gradual change from an overdrive voltage to the normal voltage. The initial value voltage is for example at least 20, or at least 10% higher than the second voltage and the time taken to reach the second voltage comprises at least 2 s. The decrease over time during the overdrive period may be provided as:

a linearly decreasing voltage portion; or

a first linearly decreasing voltage portion and a subsequent second linearly decreasing voltage portion with different linear slopes.

This two segment design compensates for different relaxation mechanisms in the EAP actuator. The first portion may have a duration of between 0.5 and 5 s and the second portion may have a duration of between 1 and 20 s.

The step increase may be to an overdrive voltage higher than the first voltage for a duration of less than 0.5 s. This provides a higher voltage overdrive spike before an overdrive level which is then decreased more gradually.

In the invention the period of the cycle comprising the overdrive period and the holding period can be larger than a period corresponding to a resonant frequency of the electroactive polymer structure. Thus, the control is not at a resonant frequency but is at a slower frequency.

The invention can be implemented in a computer programme product comprising computer readable code stored on, storable on or downloadable from a communications network, which computer readable code, when run on a computer, causes a driver to perform the steps of the methods of the invention.

The invention provides a device having an actuator comprising an electroactive polymer structure and a driver adapted to apply a drive signal as defined herein above to the electroactive polymer structure. The device benefits from features and advantages described in relation to the above methods. It will provide improved switching response possibly in combination with retained or even improved actuation accuracy.

In the device of the invention the actuator or the electroactive polymer structure comprises electrodes connected to the driver and arranged to apply the drive signal to the electroactive polymer structure.

The device of the invention can comprise a memory storing a lookup table listing a plurality of switching action data entries each data entry relating to a particular switching action form a first actuation state to a second actuation state and providing for at least one switching response time value achievable for that switching action an overdrive signal voltage and duration. Hence, if a specific response time is requested for a particular switching action, a suitable overdrive voltage can be lookup up and used by the driver. Alternatively, if a particular overdrive voltage is mandatory achievable response times can be returned.

The device can further comprise a feedback system for:

determining an actuation state of the actuator, and

based on the determined actuation state setting and/or changing one or more of the: level, duration or waveform shape of the overdrive voltage. The feedback system may comprise a displacement sensor or a closed loop driver control for regulating the drive signal parameters such as voltage levels and duration.

Hence real time control of the electrical drive signal and overdrive can be achieved giving optimum switching speed and or accuracy improvement.

The device of the invention can comprise a memory storing a computer program product of claim 11 and a microprocessor for executing the stored computer program product. Such memory can be an electrical memory in the form of RAM or ROM.

The driver device may be adapted to:

provide the first electrical drive signal as a rectangular waveform on top of the second electrical drive signal; or

provide the first electrical drive signal as a waveform on top of the second electrical drive signal with a step at the beginning and a sloped tail at the end.

These signals preferably are voltage signals. The rectangular waveform can be called an overdrive signal or overdrive voltage. There are thus various possible shapes for the first electrical drive signal.

There are thus different ways to use the overdrive function, depending on the desired speed of response, maximum voltage across the capacitance of the device, or required mechanical response.

The electroactive actuators of the invention generally are electrically driven either through electric field dependence or charge (ionic) or current dependency. By way of example of operation of an EAP actuator, field-driven EAPs are actuated by an electric field through direct electromechanical coupling. They usually require high fields (volts per meter) but low currents. Polymer layers are usually thin to keep the driving voltage as low as possible.

Examples of field-driven EAPs include Piezoelectric polymers, Electrostrictive polymers (such as PVDF based relaxor polymers) and Dielectric Elastomers. Other examples include Electrostrictive Graft polymers, Electrostrictive paper, Electrets, Electroviscoelastic Elastomers and Liquid Crystal Elastomers.

Charge or Ionic driven EAPs are activated by an electrically induced transport of ions and/or solvent. They usually require low voltages but higher currents. They often require a liquid/gel electrolyte medium, although some material systems can also operate using solid electrolytes. Examples of charge-driven EAPs are conjugated/conducting polymers, Ionic Polymer Metal Composites (IPMC) and carbon nanotubes (CNTs). Other examples include ionic polymer gels.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with reference to the accompanying schematic drawings, in which:

FIG. 1 shows a known EAP actuator, which is unconstrained and thus expands in plane;

FIG. 2 shows a known EAP actuator, which is constrained and thus deforms out of plane;

FIG. 3 shows an equivalent circuit of an EAP actuator;

FIG. 4 shows a conventional drive scheme and a drive scheme in accordance with a first example; X-axis: 0 to 500 milliseconds with 50 ms/Div and Y axis: 0 to 270 Volts with 30 V/Div.

FIG. 5 shows a conventional drive scheme and a set of drive schemes in accordance with a second example. X-axis: 0 to 800 milliseconds with 50 ms/Div and Y axis: 0 to 330 Volts with 30 V/Div;

FIGS. 6A and 6B shows driving signals for switching from low to high actuation state and vice versa, respectively. The vertical axis are voltages while horizontal axes are in time units;

FIG. 7 shows an EAP actuator system including a driver and optional actuation and control feedback system.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides a method of driving of an electroactive polymer (EAP) actuator and provides an EAP actuator capable of performing or adapted to perform the method. With the invention an adjusted drive scheme is used to faster change the actuation from one actuation state (the first or start actuation state) to another actuation state (the second or desired actuation state). The method is based on applying a sort of overdrive drive signal before providing a final drive signal associated with the ‘another’ actuation state.

The first actuation state can be a rest state (also referred to as a non-actuated state) while the desired state can be an actuated state or vice versa. The first actuation state can thus be an actuated state while the desired state can be an even further actuated state. However, as said, the desired actuation state can also be a less actuated state. The invention will improve switching from one to another state in either situation The temporary overdrive signal will improve the speed of response of the EAP structure (and thus actuator) without damaging the EAP structure and can do so without substantial actuation overshoot.

Typically an EAP actuator and its EAP structure include electrodes for receiving an electrical drive signal supplied to it by a driver. The driver can therewith control the actuator. The driver usually includes a driving circuit for providing the required electrical drive signal to the electrodes. The electrical drive signal can include or be a voltage driven signal or a current driven signal, which requires either a voltage driver or a current driver.

When the EAP structure is being activated, the driver applies (or even generates) a voltage amplitude (alternating such as AC, slowly varying, quasi static, or static such as DC) to the electrodes to therewith bring the EAP structure into the desired actuation state (e.g. actuation position).

Both EAP structures and Electronic driving circuits are not ideal. On one hand an electronic driving circuit always has internal resistances. The actuation response of an EAP actuator is therefore not only a function of the EAP structure itself but also of the driving circuit. In order to reduce the impact of the driver, the operating voltage for the EAP is usually stored in a capacitor, parallel to the EAP, and in terms of actuation this stored voltage is fed by an electronic switch (e.g. transistor, MOSFET) to the EAP actuator.

On the other hand, since an EAP actuator behaves as an electrical load with an impedance for the driver (voltage or current driver), upon setting a certain voltage or current by the driver, the voltage difference developing across the actuator electrodes usually is not entirely in sync with the setting of the signal. More specifically, and with reference to FIG. 3, from an electrical point of view, an EAP actuator (such as the one of FIG. 1) can be described as giving rise to a series connection of a resistor R_(s) and a capacitor C1, both in parallel with a further resistor R_(p). This, so called, equivalent RC circuit 30 describing the EAP actuator, is then connected to a driver 31 through points 33 and 34. While other RC circuits could be used for describing an EAP structure, the one of FIG. 3 describes an EAP to the first order quite well. The EAP structure deforms as a function of the electric field within the capacitor and thus as a function of the charge on this capacitor which again depends on the applied voltage amplitude provided by the driver 31. If the EAP is being deactivated, the applied voltage can be disconnected and accordingly the EAP will slowly discharge via its internal parallel resistance Rs and finally will go back to its initial position. However, other discharge methodologies can be applied in specific circumstances such as providing other voltage amplitudes as will be further described herein below.

Essentially the RC series circuit defines the electrical time constant τ=R_(s)·C1 (in seconds) which is an important parameter describing the temporal behaviour of such a configuration. As said, the mechanical displacement (i.e. movement) of an EAP actuator is related to the charge Q on the capacitor C1, which is defined by the applied voltage V1 and the capacitance itself (Q=C·V). Since the capacitance of the capacitor is a ‘fixed’ component with a fixed capacitance which depends on the design and construction of the actuation structure of the device (i.e. although the capacitance varies somewhat during driving, it is in first instance defined by the design configuration and EAP used), the applied voltage is the dominant parameter describing the mechanical deformation of an EAP at a steady state.

Before a steady state is reached however, the charge stored on the capacitor C1 (and thus the voltage over the capacitor) determines the instantaneous level of actuation/displacement. While the voltage V1 provided by the driver between connections 33 and 34 is used to drive the EAP structure (equivalent circuit), it is the voltage across the capacitor C1, between connections 33 and 35 that determines the level of actuation or displacement. Thus, it is an important notion that upon switching of an EAP structure to a desired actuation level, the mechanical response of the EAP structure will not overshoot the desired actuation level if an overdrive voltage V1 is applied to the structure so long as the voltage across the capacitor remains below the voltage corresponding to the desired actuation level.

The higher overdrive voltage will have a limit since there will be a breakdown voltage for the device even with the capacitance discharged. Preferably voltages applied with the invention are below the breakdown voltage.

To achieve a desired actuation level of an EAP actuator, the voltage across the capacitor and thus between nodes 33 and 35 needs to reach a certain level or amplitude. Ideally this voltage level or amplitude is reached without any (noticeable) delay. However, due to the series resistance R_(s) a delay will be introduced. For this reason, the whole charge Q required for the desired position is not built up immediately on the EAP structure, whereby the EAP structure does not directly start to actuate towards its final level with maximised speed. Instead, it heads towards a less deformed final state determined by the instantaneous charge level and with a sub optimised speed.

The invention provides an approach by which the desired charge level on the EAP is built up more quickly without disturbing reaching of the actual actuation level in an unwanted way (e.g with substantial overshoot). In order to accelerate the mechanical response time of the EAP structure, it is driven for a certain time at a higher voltage than the normal operation voltage (steady state voltage) required to reach the desired actuation position, so that the charging of the capacitor will be accelerated and hence the EAP heads towards an actuation state determined by this higher instantaneous charge level. As such it reacts faster than it would have done if operated only at its nominal operating voltage the voltage needed to reach the desired actuation state. This approach can be used to increase an actuation level more quickly, i.e. from rest or from a start actuation to a higher actuation.

When the envisaged final position is reached or about to be reached i.e. when the correct amount of charge Q associated with the desired displacement is present or almost present on the EAP capacitor C1, the driving voltage amplitude can be changed (e.g. reduced or increased) to a voltage to hold a constant position of the EAP actuator.

Without wanting to be bound by theory, in general, if a voltage V₀ is applied to a capacitor, the charging voltage v_(c)(t) as a function of time t is:

$\begin{matrix} {{v_{c}(t)} = {V_{0} \cdot \left( {1 - e^{- \frac{t}{\tau}}} \right)}} & {{Eq}.\mspace{14mu} 1} \end{matrix}$

The time constant τ is given by the product of R_(s) and C1.

τ=Rs·C1  Eq. 2

Thus, the charging voltage across the capacitor C1 reaches 63.2% of its final amplitude within the time constant τ and 99.3% after 5 times T.

Assuming a required voltage amplitude V₀ to reach a certain position of an EAP actuator, and further assuming that a higher voltage is applied, which can be considered to be an overdriving voltage V_(od), to the EAP actuator, the capacitor voltage follows:

v _(c)(t)=V ₀ =V _(od)·(1−e ^(t/τ))  Eq. 3

Finally, if the EAPs actuation position should be reached during a predetermined time t, the applied overdriving voltage can be calculated to be:

$\begin{matrix} {V_{od} = \frac{V_{0}}{\left( {1 - e^{- \frac{t}{\tau}}} \right)}} & {{Eq}.\mspace{14mu} 4} \end{matrix}$

In a similar way, if the overdriving voltage V_(od) is known, it can be estimated when, i.e. how fast, the EAP actuator will reach its required actuation position:

$\begin{matrix} {t_{od} = {{- \tau} \cdot \left\lbrack {\ln \left( {1 - \frac{V_{0}}{V_{od}}} \right)} \right\rbrack}} & {{Eq}.\mspace{14mu} 5} \end{matrix}$

A typical EAP actuator for example comprises a relaxor ferroelectric material or elastomeric system between electrodes. The equivalent component values for the circuit of FIG. 3 can be:

C1=800 nF

Rs=80 kΩ

Rp=10 MΩ

This gives a time constant of 0.064 s, so that five time constants (to reach driving displacement of 99.3%) corresponds to 0.32 s.

For implementing the overdriving principle of the invention, several driving topologies will be known by those skilled in the art. Accordingly, in the following circuit simulation an ideal and programmable voltage source is assumed.

As shown above, a required overvoltage can be determined if a certain EAP structure response time is requested.

In the following examples, actuation of an EAP structure is considered to the displacement state (desired actuation state) corresponding to a steady state voltage of 250 V. Also, the start actuation state is the rest state at zero volt. In a situation where the rest state is a non-zero volt state, then the overdrive voltage is to be added to or subtracted from the start state voltage level in order to determine the actual voltage applied to the actuation device. This is because the overdrive voltage is related to a voltage amplitude step in the drive signal.

Thus, for example, if a response time of 0.2 s is to be accomplished, the corresponding overdrive voltage can be calculated based on Eq. 4.

In this case, with an EAP structure having the equivalent components as provide above, an overdrive voltage (a first voltage) of 261.5 V is provided for a time slot of 0.2 s, after which the applied voltage is reverted to the steady state voltage (a second voltage) of 250 V.

The applied voltages and voltages over the effective capacitance of the EAP structure are shown in FIG. 4, which shows the curves 40 and 44 of the voltage across (e.g. between nodes 33 and 35 in FIG. 3) the capacitor C1 (which voltage corresponds to the actuator displacement level; see herein above) as well as the drive voltages 42 and 46 as applied between nodes 33 and 34.

Plot 40 is the conventional response to a constant drive voltage 42 (in this case being the steady state voltage of 250 V) in following an initial step increase at time zero (with the EAP actuator in an initially discharged state corresponding to the rest or non-actuated state). Plot 44 is the response to a drive voltage having a higher first voltage 46 of 261.5 V for the first period, which in this example is 0.2 s after which period the voltage level is reverted to the steady state value of 250V. While usually the capacitor is fully charged after about 5τ (=0.32 s), the EAP actuator now reaches the required charge (and therefore voltage amplitude) after the defined 0.2 s. This amounts to a 30% decrease of response time.

In one set of examples, the invention relates to near dc driving of the actuator. What is meant by this is that the actuator is held at a dc level for a period which is comparable to or longer than the actuation time. Thus, the second drive voltage is maintained for at least as long as the first drive voltage. For the example of FIG. 4, the second voltage is held for at least 0.2 s. In this case, the overall duration of the actuation is at least 0.4 s, giving a maximum frequency of operation of 2.5 Hz.

The second drive voltage may be held for at least twice as long as the first drive voltage. In this case, the overall duration of the actuation is at least 0.6 s, giving a maximum frequency of operation of 1.7 Hz.

Thus, this aspect relates to slow, near dc, actuation. For example, the maximum frequency of operation may be below 10 Hz, for example below 5 Hz or even constant voltage.

FIG. 4 shows that the voltage across the capacitor C1 (which is an effective capacitance of the EAP actuator) is at or below the second drive voltage 42 while the EAP actuator is charging (during the voltage build-up of plot 40). In this way, the second drive voltage 42 is never exceeded across the capacitor C1 so that there is no overshoot of the mechanical response. Hence accurate positioning of the actuator can be reached and with faster response time.

As explained above, if the overdriving voltage is known (or e.g. limited by the maximum voltage applicable to the overall EAP actuator to prevent breakdown), the moment in time (t_(active)), when the EAP actuator reaches its required position (i.e. its corresponding charge) can be calculated according to Eq. 5 as well. This moment in time, can also be described as a dependency of the time constant τ of the EAP actuator itself:

t _(active) =n·τ  Eq. 6

Accordingly, if the allowable amplitude of the overdrive voltage is known, the parameter n, when the EAP actuator reaches its requested position (charge) can be defined. For an overdrive voltage of V_(od)=300 V the response of the EAP actuator as modelled in these simulations is shown in FIG. 5.

FIG. 5 shows a set of different time durations for the overdrive voltage 46 and a corresponding plot 44 for each. The plots 44 are labelled with the respective values of n (which are n=0, 1.0, 1.5, 1.8, 2.0, 3.0, 5.0). The overdrive voltage of 300 V for the respective voltage pulses 46 can be read from the FIG. 5 from the vertical lines marking the end of each of the overdrive pulses 46. The overdrive voltage for example needs to be applied for 0.115 s in order to reach the requested actuation position (i.e to reach the requested charge on and voltage over the effective capacitor C1). This corresponds to a parameter n=1.8, which means that the EAP-position can be reached faster by a factor of 0.32/0.115=2.78 than when only the steady state voltage of 250 volts would have been used.

As can be seen from FIG. 5, if the period of time during which the overdrive voltage is applied to the EAP actuator is too short (e.g. n=1.0 or n=1.5), the response can be accelerated as well, but it is no longer optimal. On the other hand side, if the period of time during which the overvoltage is applied to the EAP actuator is too long (e.g. n=3.0 or n=5.0), the EAP actuator is over-driven and it takes even longer than the period of 5T to reach the desired position because now the overshoot needs to be corrected. Note that this is a mechanical overdrive hence if mechanical output needs to be precise, this can be disturbing. The invention can prevent such mechanical overdrive.

This mechanical overshoot may however be used to advantage. For example, if the EAP actuator is being used as a form of haptic user interface, a response with slight overshoot may increase the effectiveness of the interaction whilst retaining a relatively smooth profile of the interface in steady state.

The first drive voltage may thus be applied until the voltage across the effective capacitance of the EAP actuator exceeds the second drive voltage by a predetermined amount, such that the voltage drops back to the second drive voltage subsequently. Such predetermined amount can be e.g. 50%, 20%, 10% 5%, 2%, 1%.

In such a case, the (intentional) overshoot may also be followed by a period of intentional lower applied voltage to decrease the time from the overshoot back to the desired steady state after the initial intentional overshoot. In this way, there e.g could be an increased sensation of the haptic response (a spike) but the response time is reduced. The lower voltage may thus comprise a third voltage, which is lower than the second voltage, and is applied between the first and second voltages.

Even in this case, there is a period of actuation at the desired end voltage preferably of a duration equal to or longer than the duration of the initial overdrive voltage. Thus, the haptic interface is still a low frequency operation.

This low frequency operation (less than 10 Hz as mentioned above) is far lower than a resonant frequency behaviour of the device. This resonant frequency typically ranges from about 40 Hz to about 60 Hz for a free standing device or for single edge clamped device (with typical length of about 10 mm) but could be 200 Hz to 400 Hz for a system clamped on both edges. The resonance frequency will depend on the design of the device.

As can be seen from the above investigations, the response time of an EAP actuator can be improved enormously by applying, for a certain period of time, a higher voltage to the EAP actuator than that required to reach a required position in steady state.

Also, it is known that the maximum applicable voltage or breakdown voltage of all polymers (so including EAPs) is time dependent. Thus, an EAP actuator can resist a higher maximum voltage for a short period of time. Therefore, the overdrive voltage can temporarily be increased above the maximum (long-term) working voltage of the EAP device without the risk of premature breakdown. In the above example, the maximum working voltage level or amplitude for the EAP actuators is 250 V, but for short periods of time, these EAP actuators can withstand higher voltages (up to 350 V for time periods in the 10-100 ms range).

Hereinabove, the C1 was assumed to be have a constant capacitance. It appears however that for many practical EAP actuators its value varies not only during switching, but is also different for different actuator states. Hence, in going from one state to another there may be and usually is a (significant) change of C1 capacitance. This is largely due to actuator design factors that change via change of capacitor geometrics (e.g. capacitor dielectric which includes the EAP layer) thickness may change and/or the electrodes area may change) and/or via change of capacitor dielectric material properties such as the dielectric constant. The latter effect is for example seen with ferroelectrics. The invention can take account of these changes. One method is to determine the overdrive voltage or the speed of switching based on the highest effective capacitance of the states between which is switched. Often the most actuated state needs the highest voltage amplitude and therewith has the highest capacitance associated with it. Hence the C can be determined for anyone number of actuation voltage (steady state) levels in order to be used for any one of the calculations above.

The equivalent component values used above and those for any practical system can be determined using methods for impedance measurement as known in the art. The various RC models such as the one of FIG. 3 can be fitted to the results of such measurements to determine the actual resistance and capacitance values of the practical system. The effective capacitance can be the value observed at steady state as then charging has ceased and a voltage is applied over the capacitance value.

The invention can be applied for switching of an actuator from lower actuation state (such as e.g. a rest state) to a more actuated state or vice versa. FIG. 6A shows an example drive signal waveform 60 for switching an actuator form a first actuation state associated with the first voltage 61 to a second actuation state associated with a second voltage 67. Thus, at first the actuator is driven with a first voltage 61 corresponding to a low actuation state. In this case the voltage 61 is higher than zero volt as zero volt is at level 62 and thus the first actuation state is in this case not the rest state. At time 63 overdrive period 64 starts and the drive signal voltage is increased to the overdrive voltage 65 by a change of voltage 66 where this change of voltage is larger than the difference between the second voltage 67 and first voltage 61 (the difference being defined as voltage 67 minus the voltage 61). Note that the change is a positive value as the voltage difference is positive. During the overdrive period 64, the overdrive voltage is maintained at constant level to be reduced to the second voltage 67 at its end at time 68. The second voltage 67 is maintained constant during the holding period 64″.

In the switching situation of FIG. 6As, voltages were both positive and increasing for overdriving as switching occurred from a lower voltage state to a higher voltage state of an actuator. The invention works however in a similar way for the reverse situation where the same actuator is switched from higher actuation state to a lower actuation state.

Thus, FIG. 6B shows an example drive signal waveform 60′ for switching the actuator of FIG. 6A from a first actuation state associated with the first voltage 61′ to a second actuation state associated with a second voltage 67′. Thus, at first the actuator is driven with a first voltage 61′ corresponding to a high actuation state. At time 63′ overdrive period 64′ starts and the drive signal voltage is decreased to the overdrive voltage 65′ by a change of voltage 66′ where this change of voltage is larger than the difference between the second voltage 67′ and the first voltage 61′ and (the difference being define as voltage 67′ minus the voltage 61′). Note that the change is a negative value as the difference between the voltage levels is negative. During the overdrive period 64′, the overdrive voltage is maintained at constant level to be increased to the second voltage 67′ at its end at time 68′. The holding voltage 67′ is maintained constant during the holding period 64′. In extreme cases the overdrive voltage 65′ may even change sign so that a negative voltage is applied for example right up to the point where the voltage across the capacitance approaches the second voltage.

The examples of FIGS. 4 and 5 above are based on a rectangular pulse-shaped overdrive voltage, which may be considered to be a rectangular overvoltage superposed over the normal drive voltage. The duration may be variable and may be related to the electrical time constant defined by the EAP structure and or actuator itself. The amplitude of the overdrive voltage may be variable as well and may even exceed a maximum working voltage amplitude of the EAP actuator due to the short nature of the overdriving pulse.

Non-rectangular voltage waveforms are possible. For example, the overdrive voltage may be reduced in amplitude or increased in amplitude over time. Ideally the overdrive voltage amplitude will be reduced to the nominal operation voltage when the required position of the EAP is reached (i.e. if the required charge level to reach the envisaged position is reached).

Thus, for example in period 64 of FIG. 6A, the overdrive voltage is kept at value 65 giving a rectangular shaped waveform. In an alternative waveform shape, the overdrive voltage changes from value 65 to value 67 within the period 64. The change is e.g. linear with waveform 69 and stepwise with waveform 69′. Other shapes can be used. The voltage change 66 can occur using a steep rise or decrease of voltage which is linear or not. However less steep changes can be used. This may come with a loss of maximum switching speed increase however.

The accelerated behaviour as well as the transient response of the EAP device (i.e. the deformation the EAP will undergo during a certain period of time) can be influenced by the shape of the voltage applied to the component. Instead of applying an overdrive voltage with constant amplitude to the EAP actuator, any other linear or non-linear voltage form—with any arbitrary shape may be applied. In particular waveforms like multiple stepwise rectangular pulses (with the same or different duty cycles), exponential and logarithmic based profiles may be used, or combinations of these. This includes stepwise linear and non-linear voltage waveforms.

In general, the integral area under the voltage/time curve exceeds that of the steady state voltage and many different waveform shapes may be used.

An electronic driver may be used to vary the output voltage as a function of time in the desired manner. This includes open loop control for example using a look-up table of overdrive voltage amplitudes and time slot lengths. Alternatively, closed loop control may be used, having feedback sensor based settings. The feedback sensors may provide electrical, mechanical or optical feedback. A small camera may be used to provide the feedback.

Since the EAP actuators mechanical response is not only defined by its electrical performance (other delays and dead-times may be added to the electrical delay) the applied overdrive voltage level and shape may be correlated and adapted to the combination of the total delay. Alternatively, the mechanical or optical feedback may be used to correlate the mechanical response to the electrical performance of the EAP actuator.

FIG. 7 shows a driver 100 used to apply the drive signal (again in this case a voltage) to the EAP actuator 102. It also shows the optional feedback path 104 (mechanical, optical or electrical) that can be used in the invention.

The driver can include an electronic circuit with conventional PCB and discrete electrical elements. Alternatively it can be a semiconductor implemented device such as an IC as known in the art. The driver can be configured as a switching device using an external power source and from that generate and or switch power signals to the electroactive polymer structure. It may also include the power source such as a voltage source or a current source. There may also be a computer or CPU for controlling the electrical circuit of the driver. The drive signal usually is provided as a voltage to the electrodes of an EAP structure, such that it generates a voltage difference between them. A reference voltage such as zero volt may be used to ground one of the electrodes. The computer can be implemented in the driver, but can also be remotely connected (using standard wired or wireless connections as known in the art). Software stored (in a computer memory of any known type) or running on the computer can be present having code that makes the driver implement the driving. The software can have code to allow a user to provide parameter values. The software can be stored on a computer readable medium such as electronic memory as known in the art such as e.g. RAM or ROM, FLASH, SD etc., or on magnetic memory such as e.g. HDD and the like, or optical memory such as CD, DVD, Blu ray etc. or other. Alternatively, the software can be operable through or downloadable from a communications network such as 3G or 4G, LAN WAN, wired or wireless networks as e.g. known in the art.

The CPU including the memory may be located in a sub-device separate from a further sub-device that includes the actuator and thus the EAP structure. Both sub-devices thus being part of the device according to the invention. There are then also communication units such as wired, or wireless data transmitters and/or receivers located in the sub-device and further sub-device to allow the CPU to communicate with the driver to therewith implement remote control of the actuator. The sub-device may be a handheld control device of any sort, being dedicated for an application or general such as mobile phone, wearable device or similar device.

This invention relates in particular to actuation of EAP actuators comprising EAP as part of an EAP structure. The EAP structure thus comprises an EAP material. This is a material that can make the EAP structure deform upon providing an electrical signal to the EAP structure. As such the EAP material can be a mixture (homogeneous or heterogeneous) comprising or consisting of one or more matrix materials with one or more EAPs. This can for example be an EAP dispersion in a further polymer matrix material. The further polymer matrix material can be a network polymer that allows deformation invoked by the EAP mixed in or dispersed within the matrix network. The EAP material can be dispersed in it. Elastic materials are examples of such networks. Preferably the amount of EAP in such composite EAP materials is chosen from the group consisting of >50 weight or mole percent, >75 weight or mole percent or >90 weight or mole percent. EAP materials can also comprise polymers that contain in their molecules parts of EAPs (or EAP active groups) and parts of inactive other polymers. Many electroactive polymers can be used a number of which will be described below.

Within the subclass of field driven EAPs, a first notable subclass of field driven EAPs are Piezoelectric and Electrostrictive polymers. While the electromechanical performance of traditional piezoelectric polymers is limited, a breakthrough in improving this performance has led to PVDF relaxor polymers, which show spontaneous electric polarization (field driven alignment). These materials can be pre-strained for improved performance in the strained direction (pre-strain leads to better molecular alignment).

Another subclass of field driven EAPs is that of Dielectric Elastomers. A thin film of this material may be sandwiched between compliant electrodes, forming a capacitor such as a parallel plate capacitor. In the case of dielectric elastomers, the Maxwell stress induced by the applied electric field results in a stress on the film, causing it to contract in thickness and expand in area. Strain performance is typically enlarged by pre-straining the elastomer (requiring a frame to hold the pre-strain). Strains can be considerable (10-300%). For this class of materials, electrodes are preferably mechanically attached either directly or with intermediate material layers to the EAP material.

For the first subclass of materials normally thin film metal electrodes are used since strains usually are in the moderate regime (1-5%), also other types of electrodes, such as e.g. conducting polymers, carbon black based oils, gels or elastomers, etc. can also be used. For the second class of materials typically type of electrode materials is constrained by the high strains. Thus for dielectric materials with low and moderate strains, metal electrodes and conducting polymer electrodes can be considered, for the high-strain regime, carbon black based oils, gels or elastomers are typically used.

A first notable subclass of ionic EAPs is Ionic Polymer Metal Composites (IPMCs). IPMCs consist of a solvent swollen ion-exchange polymer membrane laminated between two thin metal or carbon based electrodes and requires the use of an electrolyte.

Typical electrode materials are Pt, Gd, CNTs, CPs, Pd. Typical electrolytes are Li+ and Na+ water-based solutions. When a field is applied, cations typically travel to the cathode side together with water. This leads to reorganization of hydrophilic clusters and to polymer expansion. Strain in the cathode area leads to stress in rest of the polymer matrix resulting in bending towards the anode. Reversing the applied voltage inverts bending. Well known polymer membranes are Nafion® and Flemion®.

Another notable subclass of Ionic polymers is Conjugated/conducting polymers. A conjugated polymer actuator typically consists of an electrolyte sandwiched by two layers of the conjugated polymer. The electrolyte is used to change oxidation state. When a potential is applied to the polymer through the electrolyte, electrons are added to or removed from the polymer, driving oxidation and reduction. Reduction results in contraction, oxidation in expansion.

In some cases, thin film electrodes are added when the polymer itself lacks sufficient conductivity (dimension-wise). The electrolyte can be a liquid, a gel or a solid material (i.e. complex of high molecular weight polymers and metal salts). Most common conjugated polymers are polypyrolle (PPy), Polyaniline (PANi) and polythiophene (PTh).

An actuator may also be formed of carbon nanotubes (CNTs), suspended in an electrolyte. The electrolyte forms a double layer with the nanotubes, allowing injection of charges. This double-layer charge injection is considered as the primary mechanism in CNT actuators. The CNT acts as an electrode capacitor with charge injected into the CNT, which is then balanced by an electrical double-layer formed by movement of electrolytes to the CNT surface. Changing the charge on the carbon atoms results in changes of C—C bond length. As a result, expansion and contraction of single CNT can be observed.

In relation to the above materials and with more detail, electro-active polymers thus can include, but are not limited to, the sub-classes: piezoelectric polymers, electromechanical polymers, relaxor ferroelectric polymers, electrostrictive polymers, dielectric elastomers, liquid crystal elastomers, conjugated polymers, Ionic Polymer Metal Composites, ionic gels and polymer gels.

The sub-class electrostrictive polymers includes, but is not limited to: Polyvinylidene fluoride (PVDF), Polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), Polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (PVDF-TrFE-CFE), Polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) (PVDF-TrFE-CTFE), Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyurethanes or blends thereof.

The sub-class dielectric elastomers includes, but is not limited to: acrylates, polyurethanes, silicones.

The sub-class conjugated polymers includes, but is not limited to:

polypyrrole, poly-3,4-ethylenedioxythiophene, poly(p-phenylene sulfide), polyanilines.

Ionic devices may be based on ionic polymer-metal composites (IPMCs) or conjugated polymers. An ionic polymer-metal composite (IPMC) is a synthetic composite nanomaterial that displays artificial muscle behavior under an applied voltage or electric field.

In more detail, IPMCs are composed of an ionic polymer like Nafion or Flemion whose surfaces are chemically plated or physically coated with conductors such as platinum or gold, or carbon-based electrodes. Under an applied voltage, ion migration and redistribution due to the imposed voltage across a strip of IPMCs result in a bending deformation. The polymer is a solvent swollen ion-exchange polymer membrane. The field causes cations travel to cathode side together with water. This leads to reorganization of hydrophilic clusters and to polymer expansion. Strain in the cathode area leads to stress in rest of the polymer matrix resulting in bending towards the anode. Reversing the applied voltage inverts the bending.

The electrodes of the EAP structure can have many configurations each with specific advantages and effects.

If the plated electrodes are arranged in a non-symmetric configuration, the imposed signals (e.g.voltage) can induce all kinds of deformations such as twisting, rolling, torsioning, turning, and non-symmetric bending deformation of the EAP structure.

In all of these examples, additional passive layers may be provided for influencing the electrical and/or mechanical behavior of the EAP material layer in response to an applied electric field or current.

The EAP material layer of each unit may be sandwiched between electrodes. Alternatively, electrodes can be on a same side of the EAP material. In either case, electrodes can be physically attached to the EAP material either directly without any (passive) layers in between, or indirectly with additional (passive) layers in between. But this need not always be the case. For relaxor or permanent piezoelectric or ferroelectric EAPs, direct contact is not necessary. In the latter case electrodes in the vicinity of the EAPs suffices as long as the electrodes can provide an electric field to the EAPs, the EAP structure will have its actuation function. For the dielectric elastomers it is electric field generated mechanical force between electrodes that causes the deformation. Hence the electrodes need to be physically attached either directly or indirectly (e.g. with layers between the electrodes and the EAP) to the EAP. The electrodes may be stretchable so that they follow the deformation of the EAP material layer. Materials suitable for the electrodes are also known, and may for example be selected from the group consisting of thin metal films, such as gold, copper, or aluminum or organic conductors such as carbon black, carbon nanotubes, graphene, poly-aniline (PANI), poly(3,4-ethylenedioxythiophene) (PEDOT), e.g. poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). Metalized polyester films may also be used, such as metalized polyethylene terephthalate (PET), for example using an aluminum coating.

An EAP structure may thus be used for actuation. Alternatively or additionally it may be used for sensing. The most prominent sensing mechanisms are based on force measurements and strain detection. Dielectric elastomers, for example, can be easily stretched by an external force. By putting a low voltage on the sensor, the strain can be measured as a function of voltage (the voltage is a function of the area). Direct voltages can be created by mechanical deformation of a piezoelectric EAP. Piezoelectric and electrostrictive polymer sensors can generate an electric charge in response to applied mechanical stress (given that the amount of crystallinity in the piezoelectric EAP is high enough to generate a detectable charge). Conjugated polymers can make use of the piezo-ionic effect (mechanical stress leads to exertion of ions). CNTs experience a change of charge on the CNT surface when exposed to stress, which can be measured. It has also been shown that the resistance of CNTs change when in contact with gaseous molecules (e.g. O₂, NO₂), making CNTs usable as gas detectors. Another way of sensing with field driven systems is measuring the capacitance-change directly or measuring changes in electrode resistance as a function of strain.

The invention can be applied in many EAP actuator systems, including examples where a matrix array of actuators is of interest.

In many applications the main function of the product relies on the (local) manipulation of human tissue, or the actuation of tissue contacting interfaces. In such applications EAP actuators for example provide unique benefits mainly because of the small form factor, the flexibility and the high energy density. Hence can be easily integrated in soft, 3D-shaped and/or miniature products and interfaces. Examples of such applications are:

Skin cosmetic treatments such as skin actuation devices in the form of a responsive polymer based skin patches which apply a constant or cyclic stretch to the skin in order to tension the skin or to reduce wrinkles;

Respiratory devices with a patient interface mask which has a responsive polymer based active cushion or seal, to provide an alternating normal pressure to the skin which reduces or prevents facial red marks;

Electric shavers with an adaptive shaving head. The height of the skin contacting surfaces can be adjusted using responsive polymer actuators in order to influence the balance between closeness and irritation;

Oral cleaning devices such as an air floss with a dynamic nozzle actuator to improve the reach of the spray, especially in the spaces between the teeth. Alternatively, toothbrushes may be provided with activated tufts;

Consumer electronics devices or touch panels which provide local haptic feedback via an array of responsive polymer transducers which is integrated in or near the user interface;

Catheters with a steerable tip to enable easy navigation in tortuous blood vessels.

Another category of relevant application which benefits from such actuators relates to the modification of light. Optical elements such as lenses, reflective surfaces, gratings etc. can be made adaptive by shape or position adaptation using these actuators. Here one benefit of EAPs for example is a lower power consumption.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Summarizing, an electroactive polymer actuator comprises an electroactive polymer structure and a driver for providing an actuation drive signal. In one aspect a first drive signal with an overdrive voltage is used to change the charge of the electroactive polymer structure needed for switching the structure from one to another actuation state. When or after the electroactive polymer structure actuation is near or at the another actuation state, a drive voltage is used to bring to and hold the electroactive polymer structure at the actuated state. This temporary overdrive scheme improves the speed response without damaging the electroactive polymer structure. 

1. A method of driving an actuator, wherein the actuator comprises an electroactive polymer structure, wherein the electroactive polymer structure is arranged to adopt at least a first actuation state and a second actuation state, wherein the second actuation state is different from the first actuation state, wherein the first actuation state has a first voltage associated with it and the second actuation state has a second voltage associated with it the method comprising: applying an overdrive period of a drive signal, wherein the drive signal has a drive signal voltage, wherein the drive signal voltage is applied to the electroactive polymer structure for switching the electroactive polymer structure from the first actuation state to the second actuation state, wherein the overdrive period comprises a change the drive signal voltage from the first voltage to an overdrive voltage by an amount that exceeds the difference between the second voltage and the first voltage; and applying a holding period of the drive signal, wherein the drive signal voltage is at the second voltage at least at the beginning the holding period.
 2. The method as claimed in claim 1, wherein the second voltage is applied only when or after the electroactive polymer structure reaches the second actuation state for the first time during the application of the electrical drive signal.
 3. The method as claimed in claim 1, wherein the second voltage is maintained at least as long as the overdrive voltage.
 4. The method as claimed in claim 1, wherein the electroactive polymer structure comprises electrodes, wherein the electrodes are arranged to receive the electrical drive signal, wherein the electrodes define a capacitor having a capacitance, wherein the overdrive voltage is applied until the voltage across the capacitance differs from the second voltage by a predetermined amount, wherein voltage across the capacitance changes to the second voltage after the voltage across the capacitance differs from the second voltage by a predetermined amount.
 5. The method as claimed in claim 4, wherein the predetermined amount is less than 50%.
 6. The method as claimed in claim 4, wherein the overdrive voltage is applied until the voltage across the capacitance is equal to or less than the second voltage.
 7. The method as claimed in claim 4 wherein the overdrive voltage is applied until the voltage across the capacitance exceeds the second voltage by the predetermined amount, such that the voltage across the capacitance changes back to the second voltage after the voltage across the capacitance exceeds the second voltage by the predetermined amount.
 8. The method as claimed in claim 1, wherein during the overdrive period the overdrive voltage is substantially constant.
 9. The method as claimed in claim 1, wherein the combination of the overdrive period and the holding period is a cycle, wherein the period of the cycle is larger than a period corresponding to a resonant frequency of the electroactive polymer structure.
 10. A computer program product comprising computer readable code stored on a nonvolatile storage medium, wherein the computer readable code, when run on a computer, causes a driver circuit to perform the method of claim
 1. 11. An actuator device comprising: an actuator, the actuator comprising: an electroactive polymer structure, wherein the electroactive polymer structure is arranged to adopt at least a first actuation state and a second actuation state, wherein the second actuation state is different from the first actuation state, wherein the first actuation state is associated with a first voltage associated, wherein the second actuation state is associated with a second voltage; and a driver circuit arranged to apply a drive signal to the electroactive polymer structure, wherein the drive signal has a drive signal voltage, wherein the drive signal is arranged to switch the electroactive polymer structure from the first actuation state to the second actuation state, wherein the drive signal comprises an overdrive period and a holding period, wherein the drive signal voltage is changed from the first voltage to an overdrive voltage by an amount that exceeds the difference between the second voltage and the first voltage during the overdrive period, wherein the drive signal voltage is set at the second voltage at least during the beginning of the holding period, wherein the holding period occurs after the overdrive period.
 12. The actuator device as claimed in claim 11, wherein the actuator comprises electrodes connected to the driver circuit, wherein the driver circuit is arranged to apply the drive signal to the electroactive polymer structure.
 13. The actuator device as claimed in claim 11, wherein the actuator device comprises a memory storing a lookup table, wherein the lookup table lists a plurality of switching action data entries, wherein each data entry relates to a particular switching from a first actuation state to a second actuation state and providing for at least one switching response time value achievable for that particular switching of an overdrive voltage and an overdrive period.
 14. The actuator device as claimed in claim 11, further comprising a feedback system, wherein the feedback system determines an actuation state of the actuator, wherein the feedback system sets and/or changes one or more of the level, duration or waveform shape of the overdrive voltage based on the determined actuation state.
 15. (canceled)
 16. The method as claimed in claim 1, wherein the second voltage is maintained at least twice as long as the overdrive voltage.
 17. The method as claimed in claim 1, wherein during the overdrive period the overdrive voltage changes from an initial value to the second voltage
 18. The actuator device as claimed in claim 11, wherein the electroactive polymer structure comprises electrodes connected to the driver circuit, and wherein the driver circuit is arranged to apply the drive signal to the electroactive polymer structure.
 19. The actuator device as claimed in claim 11, wherein the electroactive polymer structure comprises electrodes, wherein the electrodes are arranged to receive the electrical drive signal, wherein the electrodes define a capacitor having a capacitance, wherein the overdrive voltage is applied until the voltage across the capacitance differs from the second voltage by a predetermined amount, wherein voltage across the capacitance changes to the second voltage after the voltage across the capacitance differs from the second voltage by a predetermined amount.
 20. The actuator device as claimed in claim 19, wherein the overdrive voltage is applied until the voltage across the capacitance is equal to or less than the second voltage.
 21. The actuator device as claimed in claim 19, wherein the overdrive voltage is applied until the voltage across the capacitance exceeds the second voltage by the predetermined amount, such that the voltage across the capacitance changes back to the second voltage after the voltage across the capacitance exceeds the second voltage by the predetermined amount. 